SOURCE ROCK DISTRIBUTION AND PETROLEUM ACCUMULATION IN KWANGA BASIN WEST AFRICA

The Kwanza Basin, as one of the West African passive continental margin basins, has undergone a complex history of basin development consisting of rift stage (Late Jurassic to Early Cretaceous), transitional stage (Early Cretaceous Aptian) and post-rift stage (Late Cretaceous to Holocene). A striking feature of the transitional stage is the wide deposition of the Aptian salt. The Early Cretaceous, Late Cretaceous and Paleocene sequences have been identified as major hydrocarbon source rocks. They include the Early Cretaceous Barremian sub-salt lacustrine shale, the Late Cretaceous Aptian intra-salt marine carbonates and the Paleocene supra-salt marine shale. Oil and gas fields occur in circles around the graben which were filled with source rocks, so the source rocks remain the most significant factor for petroleum formation and distribution in the Kwanza Basin. Four major petroleum systems are defined in the Kwanza Basin, i.e. the Lower Cretaceous sub-salt, Lower Cretaceous Aptian intra-salt, upper Cretaceous supra-salt and Paleocene supra-salt. There are two types of hydrocarbon accumulation models. One is the model of self-generation and self-accumulation, and the other the model of early generatation and later-accumulation. The Cameia-1, Cameia-2 and Azul-1 deepwater exploration wells, which are located in the Block 21 and 23 in the Kwanza Basin respectively, are the significant sub-salt oil's discovery in the Lower Cretaceous sequences. It proves that there are petroleum systems in the Lower Cretaceous sub-salt strata and source rocks available in the Early Cretaceous Barremian organic-rich lacustrine shale. It has provided a significant new sub-salt oil exploration target in the deep water area of the Kwanza Basin.